HIV is most frequently transmitted following sexual contact, and semen is the vehicle fueling the global spread of this deadly virus. Far from being a passive vector for HIV, our research revealed that semen drastically enhances HIV infection in vitro, and we have further identified and characterized amyloid fibrils from human semen that increase HIV-1 fusion to its cellular targets. These fibrils can enhance HIV infection by over several orders of magnitude, and therefore serve as good targets for the development of a topical HIV microbicide. The first two aims of this proposal focus on better characterizing the mechanisms by which these semen fibrils enhance HIV infectivity and their influence on cells present within the genital mucosa. In Aim 1, I propose to use techniques from the field of neurobiology to characterize the morphological characteristics of semen fibrils that most effectively enhance HIV infectivity. This information will reveal the types of amyloid conformations in semen that should be targeted in efforts to design specific inhibitors against host factors in semen. In Aim 2, I will determine whether semen fibrils induce inflammation in host cells, and what role this may have in promoting HIV infection. It is known that inflammation generally facilitates HIV transmission by recruiting susceptible target cells and promoting HIV gene transcription. Understanding the extent to which semen fibrils contribute to host inflammation during transmission will be vital for developing microbicides that are effective in preventing sexual transmission of the virus. Lastly, Aim 3 of the proposal focuses on better understanding the fundamental physiological function of semen fibrils. Semen fibrils did not evolve to promote HIV infection, and may have a biological purpose in humans. Intriguingly, HIV fusion to its cellular target shares many properties with the fusion of a spermatozoon to an egg, raising the possibility for a role for these fibrils in fertilization. In this aim, I propose in viro fertilization (IVF) and in vivo artificial insemination experiments to determine if the semen fibris we have characterized promote the fusion of murine gametes. Understanding whether these fibrils serve to promote fertilization is vital information for the development of an HIV microbicide, and could also have a significant impact in the field of reproduction. Substantial efforts have been invested into developing an effective HIV microbicide. However, the field still lacks a drug that is highly effective at preventing the sexual spread of HIV, in part due to our lack of basic understanding of the molecular events surrounding mucosal HIV transmission. This proposal focuses on better understanding one aspect of HIV transmission, namely the effect of naturally-occurring semen fibrils that enhance HIV infectivity. Although the proposal is limited to in vitro analysis of these fibrils, we are initiating experiments in parallel in rhesus macaques to examine the effect of these fibrils in vivo. My plan during the mentored phase of the K99/R00, were it to get funded, is to develop new technical skills in amyloid and reproductive biology and to apply these skills towards understanding HIV transmission. This will be accomplished by working closely with my K99 mentors and advisory committee. My long-term goal is to advance our understanding of the molecular events surrounding HIV transmission in the genital mucosa, and to translate this knowledge into the development of new classes of HIV preventatives.